1. Field of the Invention
The present invention relates to air-fuel ratio controllers for internal-combustion engines, and more particularly to an air-fuel ratio controller for an internal-combustion engine that has respective exhaust gas sensors disposed on both the upstream and downstream sides of a catalyst and controls the supply rate of a fuel in accordance with the output signals sent from the exhaust gas sensors.
2. Background Art
The apparatuses have heretofore been known which have a wide-area air-fuel ratio sensor (A/F sensor) disposed on the upstream side of a catalyst in an exhaust gas passageway and a dioxide gas sensor (O2 sensor) on the downstream side of the catalyst, and thus which control the air-fuel ratio in accordance with the output signals sent from the two exhaust gas sensors. The A/F sensor is an exhaust gas sensor exhibiting linear output characteristics with respect to an air-fuel ratio. The O2 sensor is an exhaust gas sensor exhibiting the so-called Z-characteristics with respect to the air-fuel ratio, in which case the output of the sensor suddenly changes between rich and lean sides with a theoretical air-fuel ratio as its reference. In a conventional controller having these two exhaust gas sensors, the amount of fuel injection is feedback-controlled in accordance with the output signal (air-fuel ratio signal) from the A/F sensor to ensure that the air-fuel ratio of the exhaust gases flowing into the catalyst equals a target air-fuel ratio (hereinafter, the control is referred to as the main feedback control). Along with the main feedback control, control is conducted by feeding the output signal from the O2 sensor back into the amount of fuel injection (hereinafter, the control is referred to as sub-feedback control).
Sub-feedback control is executed to complement the main feedback control and improve the emission characteristics of the internal-combustion engine. The target air-fuel ratio for use in the main feedback control is set to an air-fuel ratio at which the catalyst can purify the exhaust gases with the highest achievable efficiently, and for the main feedback control, a feedback correction value is calculated according to the particular deviation between the air-fuel ratio signal from the A/F sensor and the target air-fuel ratio. The effects of various parameter changes in the internal-combustion engine, however, may cause the actual air-fuel ratio of the exhaust gases with respect to the target air-fuel ratio to tend to be biased to the rich side or the lean side, despite the main feedback control being conducted. If such a tendency continues, the will be exhausted of occluded oxygen before too long, and hydrocarbon (HC) and carbon monoxide (CO) will become unable to be purified (if the actual air-fuel ratio tends to be biased to the rich side). Conversely, the oxygen-occluding state of the catalyst will saturate and nitrogen oxides (NOx) will become unable to be purified (if the actual air-fuel ratio tends to be biased to the lean side).
The output signal from the O2 sensor represents the oxygen-occluding state of the catalyst, and if the catalyst is exhausted of the occluded oxygen, the output signal from the O2 sensor will be a rich-state output, whereas, if the oxygen-occluding state of the catalyst saturates, the output signal from the O2 sensor will be a lean-state output. When the output signal from the O2 sensor reverses to indicate a rich state, therefore, it can be judged that the actual air-fuel ratio of the exhaust gases flowing into the catalyst is biased to the rich side. Conversely, when the output signal from the O2 sensor reverses to indicate a lean state, it can be judged that the actual air-fuel ratio is biased to the lean side.
For sub-feedback control, a sub-feedback correction value is calculated in accordance with the output signal from the O2 sensor and then the sub-feedback correction value is fed back for the main feedback control, whereby the deviation between the air-fuel ratio signal from the A/F sensor and the target air-fuel ratio is corrected. Thus, the deviation between the air-fuel ratio signal from the A/F sensor and the target air-fuel ratio can be brought close to the deviation between an actual air-fuel ratio signal and a target air-fuel ratio, and thus, control accuracy of the air-fuel ratio by the main feedback control can be enhanced.
The conventional controllers known to control the air-fuel ratio by conducting sub-feedback control together with the main feedback control include, for example, the controller disclosed in Japanese Patent Laid-open No. 8-291738. During the sub-feedback control in this device, an air-fuel ratio correction value is calculated by conducting proportional and integral control (PI control) based on the output signal sent from the O2 sensor and performing computations for a proportional term (P-term) and an integral term (I-term). Also, a weighted-averaging process is performed on this air-fuel ratio correction value and the result is calculated as an air-fuel ratio learning rate. After this, a target air-fuel ratio is corrected by adding thereto both the air-fuel ratio correction value and the air-fuel ratio learning rate to complement the main feedback control.
For the conventional controller disclosed in Japanese Patent Laid-open No. 8-291738, the air-fuel ratio learning rate is derived as a learning value from the air-fuel ratio correction value, and this air-fuel ratio correction value contains the P-term and I-term obtained by conducting PI control of the output signal sent from the 02 sensor. The I-term is a steady component indicating the steady-state deviation of the O2 sensor output signal, whereas the P-term is a variable component that varies according to a particular change in the output signal of the O2 sensor. For this reason, acquisition of the P-term to learning also causes a variable component to be included in the learning value.
Accordingly, the conventional controller has had the inconvenience in which the learning value during sub-feedback control significantly fluctuates or in which obtaining a stable learning value becomes a time-consuming operation. A significant fluctuation in the learning value or a decrease in the learning rate means that biasing of the air-fuel ratio of the exhaust gases toward the lean or rich side is allowed, thus making it impossible to maintain stable emission characteristics.